In today's radio communications systems, there exists an ever increasing need to send signalling tones in addition to, and interspersed with, voice signals. In particular, dual tone multi-frequency (DTMF) signals are commonly used to provide input for interactive telephone services. Examples include so-called touch-tone banking transactions, entering employee phone extensions, telephone calling card digit entry, etc. In each of these examples, DTMF signalling tones are transmitted while the sending unit (e.g., radio) is configured in a voice processing mode (i.e., encoding circuitry configured to process voice signals for clear reception by the target user). Unlike the so-called call-setup mode (i.e., a configuration whereby signalling tones are directly sent, without further processing), voice processing (e.g., data compression, sampling, etc.) is incompatible for use in transmitting signalling tones (e.g., DTMF signals).
In addition to the aforementioned incompatibility, disturbances in the radio frequency (RF) path-between the sending unit (e.g., subscriber radio) and the fixed end hardware--results in lost data. That is, even small discontinuities in the transmission of signalling tones might cause false tone detection at the receiving end. As an example of such a disturbance, consider a transmitting mobile radio travelling underneath a bridge, or other obstruction. Inasmuch as the RF path has been physically obstructed, the signal being transmitted might have discernable, and hence problematic, discontinuities.
Accordingly, a radio communication system which provides reliable transmission of signalling tones during a voice communication would be an improvement over the prior art. Such a radio communication system would, at least partially, resolve the problem of incompatibility between voice signalling and user entered signalling tones, thereby enhancing the utility of the system.